The present disclosure pertains to an apparatus and a method for fiber delivery of high power laser beams.
Optical fibers can be used to transmit a laser beam from a laser source to a desired location. The use of an optical fiber to transmit a laser beam from a laser source to a desired location is a significant enabler in a number of laser applications because the optical fiber offers a flexible transmission path that involves no free-space optics and which can be re-routed in real time. For example, industrial lasers operating at wavelengths that can be transmitted through silica fibers almost universally use such optical fibers to move the laser beam from the laser source to the workpiece. Often, the output end of the optical fiber is attached to a robotic arm that directs the output beam to the workpiece. Another example is in medical applications, where an optical fiber is used to transmit a laser beam through a blood vessel to a specific location where the focused beam is used to accomplish a desired medical effect. Another example appears in solid-state laser designs, where radiation from pump diodes is commonly delivered to active region of a solid-state laser via fibers or bundle of fibers. Yet another example is in defense systems, where guiding a laser beam onto the inner gimbal of a beam director is desired without having to propagate through a complex sequence of optical elements that move and guide the laser beam.
While the first three examples above are already implemented in their respective application areas, the use of optical fiber delivery of laser beams in defense systems has been limited in the past by two challenges. First, the optical fiber in conventional systems is required to have a sufficiently large core area so that it can accommodate the required average or peak power without suffering from optical damage or degradation of the laser beam by nonlinear optical processes such as stimulated Brillouin scattering (SBS) or stimulated Raman scattering (SRS). Second, if a sufficiently large core is developed, the fiber must be designed such that the input beam quality, which for many military applications must be nearly diffraction-limited, is not degraded by propagation through the fiber.
Optical fibers are widely used for laser beam delivery in the industrial laser market. These optical fibers are capable of delivering laser beams with very high powers of interest. However, these conventional optical fibers are designed to transport radiation of highly multimode industrial lasers. These conventional optical fibers necessarily have core diameters as large as 0.5 to 1 mm and a large numerical aperture (NA) 0.1-0.2, such that the fibers are also highly multi-mode. Presently, there is no practical way to maintain high beam quality when using such conventional optical fibers for beam delivery. Due to the multimode aspect of these conventional optical fibers, bending of the optical fibers results in a strong mode coupling to the higher order transverse electromagnetic modes that are guided along with the fundamental and other lowest order transverse electromagnetic modes. If the lowest-order transverse electromagnetic mode is launched in these conventional optical fibers, the lowest-order mode will lose most of its power as it feeds higher-order transverse electromagnetic modes. As a result, even if the input beam is nearly diffraction-limited and the output beam suffers only minor power loss, the output beam quality is typically greater than 50-100 times diffraction limited (XDL). Therefore, these conventional optical fibers meet power-delivery requirement, but not beam-quality delivery requirements.
Another problem with typical beam-delivery fibers for high power lasers is that as the core diameter increases to accommodate the increasingly high laser power, the fiber becomes less flexible. At some point the basic purpose of the fiber, mechanical flexibility, becomes severely restricted due to the large fiber diameter.
To overcome the above deficiency with high beam quality transportation, a large mode area (LMA) optical fiber design can be implemented. LMA optical fibers can guide a few higher-order transverse electromagnetic modes while still maintaining beam quality at or better than approximately 1.3 times diffraction-limited (XDL). A LMA optical fiber differs from the standard large-core delivery fibers by having a relatively small core diameter, between about 20 μm and about 30 μm for signal wavelengths of about 1 μm, and a reduced NA of approximately 0.06. LMA fibers must be properly coiled to maintain good beam quality. At a prescribed core diameter, coiling induces bending losses to all transverse electromagnetic modes, but the higher-order modes all suffer much greater coil-induced loss than the desired lowest-order transverse mode. Hence, coiling results in increased radiation loss for higher-order transverse electromagnetic modes, which are stripped out of the core, thereby “cleaning up” the laser beam and yielding a beam having mostly the fundamental transverse electromagnetic mode. With an optimized coil radius, the loss for the fundamental transverse electromagnetic mode remains at a low and tolerable level.
A 10 kW fiber laser that generates a laser beam to be delivered to a beam director for use in industrial applications is often based on an LMA fiber. A conventional LMA beam delivery fiber can be considered to meet the requirements of beam delivery. Indeed, since the laser beam is generated by the LMA fiber in the first place, the LMA fiber can obviously accommodate the laser power. However, there are other considerations that need to be taken into account. Among these considerations is the onset of stimulated Raman scattering (SRS) or stimulated Brillouin scattering (SBS) as the beam-delivery fiber length approaches a few meters, as is required for typical applications.
What is needed is a method and apparatus that cure the deficiencies noted above in conventional optical fibers used for delivery of high power laser beams, and that provide an optical fiber taking into account the above considerations, including SRS and SBS.